It makes sense if you put one and wire it to the generator/power plot, after all in real life you need some wires from a generator to conduits and devices to actually power things, not just a generator sitting on its own (I mostly failed physics class so take this with a grain of salt)
It turns into magic when you can build a power plot on one side of the settlement and put a power pylon on the other side. You can do that, I guess for convenience sake. Only one power pylon is needed per settlement, and plots/items can be connected to it through other plots/items (as in - pylon - wired to a plot - that's wired to another plot), I think I should clarify that.
HQ is producing next to nothing, only gets from settlements. How much it's consuming you can see on the HUD if you expand it - see here
https://wiki.simsettlements2.com/gameplay/headquarters/command-mode
(Extended Supply Meters - Daily and project usage)
As for each settlement, there is a feature for that if you go to the terminal on a city planner desk, you can get settlement reports. Don't think they are super accurate right now though, especially if you have a million settlements and they updated recently.
If a settlement produces 50 water, it probably uses most of it or sends to other settlements that lack water. HQ seems to be the lowest priority in my experience. To not worry about it I'd just build and upgrade a ton of water/food plots and eventually they have enough to give away. If settlement reports work properly you should see how much is needed and what part of it is surplus. Other than that, you're right, it's pretty hard to figure out when you have no idea how much is made and how much is needed.